What is the central diffusion law?

The central diffusion method, as the name implies, is to take a certain position as the center and spread around until the conditions are met or the boundary is reached.

Leetcode 5. Longest palindrome substring

Title Description: given a string s, find the longest palindrome substring in S. You can assume that the maximum length of S is 1000.

Example 1: input: "babad", output: "bab", note: "aba" is also a valid answer.

Example 2: input: "cbbd", output: "bb"

Solution idea: traversing s, taking each char and two midpoint of char as the center, calculating the longest palindrome string with this point as the center;

For example: the string abcba has 5 (letters) + 4 (between two letters) = 9 center points; therefore, the string with length n has 2N-1 center points. Our goal is to count the longest palindrome strings S1, S2,..., s2n-1 centered on these 2N-1 points, and pick out the longest palindrome strings globally. Retain the maximum length palindrome string index, which is recorded as left and right; return the substring with left and right as the boundary after traversal

public class Solution {
    public static void main(String[] args) {
        Solution solution = new Solution();
        String s1 = "babad";
        System.out.println(solution.longestPalindrome(s1));
        String s2 = "cbbd";
        System.out.println(solution.longestPalindrome(s2));
        String s4 = "";
        System.out.println(solution.longestPalindrome(s4));
        String s3 = null;
        System.out.println(solution.longestPalindrome(s3));
    }
    public String longestPalindrome(String s) {
        if(null == s) {
            return null;
        }
        char[] charArray = s.toCharArray();
        int length = charArray.length;
        int left = 0, right = 0, longestLength = 0;
        String longestPalindromeStr = "";
        for(int i=0; i<length; i++) {//Spread around a single character, there are n Center ( n Is the length of the input string)
            left = i;
            right = i;
            while(left >= 0 && right < length && charArray[left] == charArray[right]) {
                left--;
                right++;//cabad
            }
            if(right - left - 1 > longestLength) {
                longestLength = right - left - 1;
                longestPalindromeStr = s.substring(left + 1, right);
            }
        }
        for(int i=0; i<length-1; i++) {//Spread with two characters as the center, then n-1 Center ( n Is the length of the input string)
            left = i;
            right = i+1;
            while(left >= 0 && right < length && charArray[left] == charArray[right]) {
                left--;
                right++;
            }
            if(right - left - 1 > longestLength) {
                longestLength = right - left - 1;
                longestPalindromeStr = s.substring(left + 1, right);
            }
        }
        return longestPalindromeStr;
    }
}